1. Field of the Invention
The present invention relates to optical disk recording/reproduction apparatuses, and more particularly to an optical disk recording/reproducing apparatus that can record/reproduce information to/from optical disks having different substrate thicknesses or recording densities.
2. Description of the Background Art
In these recent years, an optical disk recording/reproduction apparatus is proposed that can read out information recorded on an optical disk of approximately 1.2 mm in thickness such as a CD (compact disk) and a CD-ROM using a semiconductor laser. In such optical disk recording/reproduction apparatuses, control is provided of focus servo and tracking servo with respect to an objective lens for pickup. A laser beam is directed to a pit train on a recording surface, whereby signals of audio, video, and data are reproduced.
During the past few years, there has been intensive efforts to increase the density for the purpose of recording motion pictures of significant duration on such optical disks. For example, a SD (super density) standard is proposed to record information of approximately 5 G bytes on one side of an optical disk having a diameter of 12 cm which is identical to that of a CD-ROM. According to this SD standard, the thickness of the optical disk is approximately 0.6 mm. One optical disk having two disk substrates of the SD standard laminated to each other on their back side surfaces can be recorded with information of approximately 10 G bytes. In contrast, a MMCD (multi media compact disk) standard is proposed that records information of approximately 3.7 G bytes on one layer of an optical disk having a diameter of 12 cm which is identical to that of a CD-ROM. According to this MMCD standard, the thickness of an optical disk is approximately 1.2 mm. An optical disk having a 2-layer structure of the MMCD standard can have information of approximately 7.4 G bytes recorded thereon.
In general, an objective lens for pickup is designed taking into account the thickness of the substrate of the optical disk of interest, the refractive index and the wavelength of the employed semiconductor laser. Reproduction of information from an optical disk having a thickness different from that of design is impossible since the spot of the laser beam cannot be focused on the recording plane of the optical disk. For example, an objective lens designed to conform to an optical disk having a substrate of 1.2 mm in thickness cannot focused the spot of a laser beam on the recording surface of an optical disk having a substrate of 0.6 mm in thickness. Information cannot be reproduced from such an optical disk.
Tanaka et al. disclose an optical head including a spherical optical element for correcting aberration caused by difference in the substrate thickness of optical disks in Japanese Patent Laying-Open No. 5-303766. This optical element functions to alter the numerical aperture (NA) of the objective lens.
An objective lens is generally shifted in a vertical direction (tracking direction) with respect to the optical axis of the laser beam according to tracking control. However, the aperture of Tanaka et al. is fixed with respect to the optical axis of the laser beam independent of the tracking control. Therefore, when the objective lens is shifted in a tracking following range similar to the case lacking an aperture, the amount of deformation of the laser beam spot directed onto a recording plane will be increased according to the offset of the objective lens with respect to the optical axis of the laser beam. This phenomenon is due to the fact that reduction in the diameter of the laser beam corresponding to the aperture causes a great deformation in the laser beam spot as if the amount of displacement of the objective lens is relatively increased.
This beam spot is deformed in the running direction of the track and also the tracking direction which is perpendicular to the running direction. Deformation in the running direction of the track will become the cause of degrading the jitter. Deformation in the tracking direction will become the cause of cross talk noise.
In a conventional apparatus shown in FIG. 1A, an aperture 9 having a complete circle of an opening shown in FIG. 1B is employed. Aperture 9 is fixed with respect to the optical axis of the laser beam. In contrast, an objective lens 5 is displaced in the tracking direction so that the laser beam spot traces the track of an optical disk 6 in fidelity. Therefore, the optical axis of objective lens 5 is often offset from the optical axis of the laser beam.
FIG. 6 is a graph showing jitter with respect to the tracking following range. As indicated by the solid circle, a conventional apparatus exhibits a sudden increase in jitter as the tracking following range of object lens 5 is increased since the offset between the center of object lens 5 and the center of aperture 9 becomes greater.
In the future, it is expected that an optical disk having a substrate thickness of approximately 1.2 mm with the current density (CD, CD-ROM), an optical disk of the MMCD standard having a substrate thickness of approximately 1.2 mm with high density, and an optical disk of the SD standard having a substrate thickness of approximately 0.6 mm with high density will be concurrently available in the market. Optical disks according to the MMCD standard and the SD standard are called "digital video disks" (DVD) to be discriminated from CDs and CD-ROMs. "MMCD" and "SD" are temporary names, and may be changed in the future. In the present application, the MMCD and the SD standards are used with the implication of defining the physical characteristics of an optical disk, such as the substrate thickness and recording density.